EFFECTS OF INBREEDING AND CROSSBREEDING. 23 



Table 4. — Linear equations representing the trend in the inbred stock relative to each 



characteristic. 



(In these equations y is the measure of the character in question and x is the number of years from 1906.) 



Size of litter y= 2. 885-0. 043a; 



Litters per year y= 4. 210— . llOa; 



Young per year y= 12. 092— . 450a; 



Percentage born alive y= 88. 27 — . 29a; 



Percentage raised of those born alive y= 92. 10 — 1. 04a; 



Percentage raised of all bom y= 81. 30 — 1. 16a; 



Birth weight of young raised (grams) y= 86. 17 — . 19a; 



Gain, to 33 days (grams) 2/=163. 70 -1. 96a; 



Weight at 33 days (grams) 2/=249. 87 -2. 15a; 



Both elements in fertility, gain, and percentage raised of those born 

 alive have shown considerable decline, while the birth weight and 

 percentage born alive have shown very little decline, according to these 

 figures. 



A downward trend can of course be interpreted in two ways. It 

 may be due in some way to the inbreeding, or it may be due in some 

 way to progressively deteriorating environmental conditions. In 

 regard to the latter hypothesis, arguments could be advanced on 

 both sides. There are, however, certain considerations which show 

 that some at least of the decline was genetic in character. 



Consider first the inequalities in the decline in different characters. 

 The record for 1914, after eight years of inbreeding, surpassed any 

 preceding year except 1910 in gain, weight at 33 days, and birth 

 weight. The records surpassed five earlier years in percentage born 

 alive, and four earlier years in percentage of the latter which were 

 raised and in total percentage raised. These points indicate that in 

 1914 environmental conditions were at least as good as in any earlier 

 year, with the probable exception of 1910. Nevertheless the average 

 size and frequency of litters in 1914 is markedly inferior to aU years 

 before 1911. .Two inbred families had become extinct by 1914, but 

 these two families (1 and 15) were the two lowest in fertility and 

 their elimination should have resulted in genetic improvement of the 

 average. Thus, whatever may have been the case with other char- 

 acters, the conclusion seems unavoidable that fertility had suffered 

 a real genetic decline after eight years of inbreeding. 



A comparison between the inbreds and controls shows that in every 

 respect the controls, born in a given year under identical conditions, 

 were more vigorous than the inbreds. The difference is most marked 

 in the case of size of litter, a result which agrees with the greater 

 decline in size of litter than in other characters. It will be noted that 

 the controls were taken from the same line-bred stock from which the 

 inbred families were mainly derived. The most vigorous guinea 

 pigs were selected in both cases. The inbred families (31 to 39) 



